1. Field of the Invention
The present invention relates to an array of optical microelements such as micro-lenses, usable for a display device including a liquid crystal display, a plasma display and an electro-luminescence display, and a method of producing such microelement array, and further, a display device using the microelement array.
2. Description of the Prior Art
Recently, as thin type display devices replacing CRT (Cathode Ray Tube), various kinds of devices have been proposed, and among them, an active matrix liquid crystal display on which thin film transistors (TFT) are formed for respective pixels, this focuses attention, in particular, on the thin type display device with large color display capacity. In such device, light from a light source is transmitted through an opening area of each pixel, so that the permeability depends on the opening factor (vignetting factor) of the pixels. When, for the liquid crystal display with very fine accuracy, very small pixels are formed in high density, an area of the TFT occupying the pixels becomes relatively large because there is a limit for reducing the size of the TFT. In other words, the opening area through which the light is transmitted becomes small (drop of opening factor), and the transmitted light is decreased. Display quality becomes poor because the display becomes dark due to the decrease in the transmitted light. As a method to solve the problem, effective usage of light, formerly absorbed by wiring needed to form the TFT and shields, is achieved by converging the light, with a lens to the opening area of the pixel. For example, in Japanese laid-open patent application No. 60-165624, there is a description that conventional machining is made on a glass substrate itself to from spherical microlenses. In Japanese laid-open patent application No. 60-264334, there is a description that the lenses are formed by creating fine patterns made of glass with melting point lower than that of the substate and deforming them with surface tension caused by melting into semi-circular shapes. Or, in Japanese laid-open patent application No. 1-189685, there is a description that microlenses are formed by pressing a thermoplastic resin to make it adhere to a polished glass substrate.
However, in the case of Japanese laid-open patent application No. 60-165624, around one million microlenses are needed for a 3 inch-sized ultra-high density liquid crystal display, but forming an extremely large number of the microlenses, totaling around one million, by such a method is very difficult, or the mass production is impossible. In the case of Japanese laid-open patent application No. 60-264334, the semi-circular is made with the surface tension caused by melting, but controlling curvature of the lens is difficult, so that the method is inferior in degree of freedom and reliability. In the Japanese laid-open patent application No. 1-189685, resin, an organic compound, is used for the portion of the microlenses. Generally, the coefficient of thermal expansion of the resin, the organic compound, is around one figure larger compared to that of the glass substrate, and degree of the expansion and contraction, due to a charge of temperature, is great. Consequently, there occur problems for the microlenses formed on the glass substrate that, with usage for a long time, positions to be kept in high precision between the pixels and the microlenses are shifted, the curvature of the lens changes or the microlens array separates from the glass substrate.